Rennet is the generic name for an enzyme that performs an essential job in cheesemaking: coagulation. Added to a vat of cultured milk, rennet kickstarts a molecular chain of events that turns the liquid into a firm gel. (There are acid-coagulated cheeses too, such as paneer and ricotta, but that’s another topic.) After this firming happens the cheesemaker cuts the gel into curds and drains off the accompanying whey (the watery by-product), leaving behind the solids. When packed into forms, these curds will eventually knit themselves together into a cheese.
How Does Rennet Work?
Stirred into milk, rennet begins a two-step process that causes the protein in milk, which is normally dispersed in the liquid, to come together and form a matrix—what we call “curd.” To understand how this happens, try to imagine the ultra-tiny world of milk protein molecules—the casein type in particular. There are four different kinds of casein molecules in milk that attach to each other, forming protein “teams” called micelles that float throughout the milk. Milk is full of these micelle orbs. What’s remarkable is that molecules of one type of casein—kappa casein—protrude like fine hairs from the surface of each micelle sphere and attract water molecules. Without this attraction the micelles would separate from the liquid matter as solids. In other words, kappa casein makes milk proteins drinkable. For coagulation to happen, rennet has to perform the first step of the process, which is to cut those kappa hairs. When the surrounding layer of kappa casein breaks down, water molecules are repelled. This allows the second phase to begin, whereby all the casein molecules naturally aggregate, forming a sponge-like mass (again, curd). If caseins were not naturally inclined to attach to each other, coagulation would never happen, no matter how much rennet was added. We can thank molecular affinities, as much as rennet, for cheese as we know it.
Where Does Rennet Come From?
The source of rennet is often of most interest to cheese lovers, since the enzyme is either sourced from animals (sometimes referred to as “traditional” rennet) or comes from three different origins considered suitable for vegetarians.
The type of rennet sourced from animals is mostly made up of a coagulating enzyme called chymosin. Naturally present in the fourth stomach, or abomasum, of an unweaned calf, kid, or lamb, chymosin essentially turns milk into a soft cheese in the stomach of these young animals so that digestion occurs more slowly and nutrients can be absorbed. So cheesemakers use the coagulating property of chymosin just as Mother Nature does—to create curd, albeit in a vat, not a stomach.
Regardless of the animal source, a little chymosin goes a long way: 1 part enzyme to 15,000 parts milk does the job of coagulation. It should be noted that these young animals are usually sold for meat, and rennet is very much a by-product. Animals are not slaughtered solely for the production of rennet.
Traditionally, in rural areas, cheesemakers would get their rennet from the local butcher or abattoir in the form of vells (abomasums inflated into large balloons to help them dry). Typically, cheesemakers would buy strips of the vell. When they needed rennet, they’d rehydrate a strip in a saline water bucket, thereby naturally extracting the enzyme.
In 1874 the Hansen Company, located in Scandinavia, isolated the enzyme from the vell and began producing rennet commercially for the cheesemaking industry. This signaled the beginning of laboratory-produced rennet, which allowed cheesemakers greater control over the end product, as the strength and quality of the rennet was much more consistent.
In much of Europe most traditional cheesemakers prefer using traditional (animal) rennet. It is their experience that vegetable or microbial rennet lends a bitter taste to the cheese over time.
Natural Vegetable Rennet
True vegetable rennet comes from plants such as cardoon thistle, butterwort, artichoke, nettles, safflower, melon, yellow bedstraw, and fig leaves. To derive the coagulating enzyme (which is not chymosin, but similar) these plants are steeped in water to make a “tea.” This liquid extract is then added to the milk. The challenge of using this coagulant is that its strength varies considerably. It requires a great deal of skill to produce a consistent cheese using natural vegetable rennet. Also, in terms of flavor, these cheeses frequently develop slightly bitter flavor as they age. These risk factors ensure that vegetable rennet is rarely used for commercial production, though traditional cheeses made with true vegetable coagulants include Queso de Sierra and Torta del Casar from Spain, Serra da Estrela from Portugal, and a few French sheep’s milk cheeses from the Pyrénées.
The 1970s saw a large increase in world cheese production that placed a demand on animal-type rennet beyond available supplies. As a result, it was discovered that two molds, Rhizomucor miehei and Rhizomucor pusillus, contained the active enzyme chymosin to coagulate milk. This rennet was embraced by the vegetarian community and was found to be particularly suited to soft and young cheeses. At a time when many small American farmstead cheese producers were just getting started and the price of animal rennet was increasing rapidly, the microbial version was a welcome option.
Advances in DNA engineering since the 1980s have made it possible to produce rennet substitutes from genetically encoded microorganisms. Natural cells extracted from the calf’s stomach are used as a template and kick-start this process; the chymosin chromosome of each cell is implanted into a yeast or bacterium culture, which is then grown, extracted, and purified in the lab. Therefore, while there is no residual animal matter in the man-made chymosin itself, it originates from the real thing. For some vegetarians this is an issue, while for other consumers, the problem is the use of GMOs in general. (Some would argue that since the original enzyme structure is not altered, this process is technically not genetic engineering.) Many cheesemakers prefer GM chymosin, saying it is much more reliable and less expensive than its traditional counterpart.
As a side note, it’s interesting to observe that in India, where the cow is sacred, rennet is extracted from a live animal. While the calf is still on its mother’s milk, a small hole is bored through its hide into the abomasum. A measure of gastric fluid is drained off, which will include the chymosin. Once processed, this “offering” from the holy beast will be used to perform the miracle of coagulation. The hole is then closed up again and the calf’s sacred status is intact—sore, but undamaged.